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Ocean Energy Budget Analysis: wind and buoyancy forcing and diapycnal mixing

Special talk given by visiting scholar

The Meridional Overturning Circulation (MOC) in the ocean plays an important role in transporting heat and nutrients, supplying oxygen to the deep ocean, and sequestering atmospheric carbon below the mixed layer. The MOC is believed to be driven by surface wind stress, heat and freshwater fluxes, tides, and other energy sources, though the degree to which each of these source terms contributes to the MOC dynamics is poorly understood. To study the interplay between surface wind and buoyancy forcing, an Available Potential Energy (APE) framework is applied to data from a global ocean model ECCO2 and a regional ocean model for the Southern Ocean (SOSE). This framework allows for partitioning of the energy budget into mean and turbulent components of the kinetic and available potential energy reservoirs and decompose adiabatic (processes without heat or matter transfer) and diabatic (processes with irreversible transfer of heat or matter) portions of the ocean circulation. While the ocean models incorporate realistic bathymetry and aspect ratios, the spatial and temporal resolutions are too coarse to resolve small scale dynamics, such that mixing and dissipation rates cannot be computed directly. To fully resolve all relevant scales, I conducted direct numerical simulations (DNS) of an idealized basin representation of the Southern Ocean and compared the ocean energy budget with the SOSE results. I find that while the Southern Ocean residual circulation is sensitive to the balance between the magnitudes of surface wind stress and buoyancy forcing, diapycnal mixing rates are primarily set by the dense water plume resulting from buoyancy loss near the pole.